U.S. SCIENTIST'S INVENTION AIDING SOVIET COMET PROBE

Back in 1983 John Simpson, the University of Chicago physicist who has had more experiments on interplanetary space probes than any other scientist in the world, was about to be left out of one of the most fantastic space missions so far, the upcoming close encounter with Halley's comet.

Simpson had invented a new supersensitive comet-dust detector he hoped the European Space Agency would put on its elaborate Giotto spacecraft, now set to skim by Halley's comet March 13.

But in 1983 when Simpson went to Holland to pitch his invention, his European colleagues said it was a great idea but that they were sorry: All the space aboard Giotto had been claimed for other experiments long ago and no one wanted to give up their slot for Simpson's box.

That seemed the end. For Simpson it was a bittersweet situation. He had an important invention but no way to use it.

The United States earlier had canceled its plans to send a space probe to visit the famous comet, a penny-pinching decision much regretted now.

Although the Soviets were planning to send two spacecraft to Halley's, hitching a ride on them appeared to be out of the question since U.S. and Soviet space cooperation came to an end in 1982 when President Reagan refused to renew the agreement.

But the Soviets wanted Simpson's detector. Unknown to Simpson, a Soviet scientist had sat in on his "chalk talk" in Holland where he described his invention.

Six weeks after returning to the Chicago campus a stunned Simpson received a telegram from the Space Research Institute in Moscow saying: "We accept your experiment to Halley's comet. How soon can you be in Moscow?"

"I was totally surprised," Simpson said. "I never proposed the detector to them and I didn't even know there was a Russian in the audience."

The Soviets wanted the detector because they needed a "pathfinder" and they realized that Simpson's invention could guide them through the potentially dangerous dust field surrounding the comet.

The Soviet probe, Vega 1, is set to zoom within 5,000 miles of the solid nucleus at the core of Halley's huge gaseous head today.

The danger to the spacecraft is the dust being kicked off the nucleus by evaporating ice as the comet is warmed by its close swing around the Sun.

The dust particles are traveling at a relative speed to the comet of about 155,000 miles an hour. At that speed a single fairly good sized dust particle carries a terrific wallop and could knock a space probe out of commission.

Vega 1 probably will be far enough away to escape the high-speed dust shrapnel, but Vega 2 is expected to buzz the nucleus at a distance of only 2,000 miles next Sunday.

By measuring the size of the dust particles and their density, Simpson's detectors will be able to tell Soviet flight directors the degree of danger faced by Vega 2. If the dust field is heavy, they can change the craft's course to avoid harm. If it is lighter, they can swing the craft closer to the comet's nucleus for a better look.

Information on the potential danger of the dust field will be radioed to Darmstadt, Germany, where European Space Agency officials will use it for making course changes for Giotto.

Giotto's course has been called a suicide mission because the craft is expected to come within 300 miles of Halley's nucleus on March 13.

Simpson said the decision to cooperate with Soviet space scientists was made after clearance was obtained from the federal government. However, since federal regulations prohibit transferring high-technology information to the Soviet Union, the detector had to be redesigned using off-the-shelf electronic equipment that could be bought in any country.